What is the theme of the story Umbrella Man by Roald Dahl?

The story Umbrella Man is all about a young innocent girl, aged about 13 years old, speaking about an encounter with an old man;who appears 'feeble' to her. He tells the narrator's mother that he has forgotten to bring his wallet, and his legs are too exhausted and weak for him to walk back home. He asks if the mother could buy the Silk Umbrella in exchange for some money.

After all this, the Mother finds out that the old man is a knave, who sold her an umbrella just because he needed money for a drink or two.

This story talks about how people get cheated by clever knaves. This story has a touch of originality.